1. Field of the Invention
The present invention relates to a hydraulic tandem pump assembly adapted for use in motor vehicles, and more particularly to a control system for a hydraulic tandem pump assembly of the type which is adapted to supply fluid under pressure to a power-assisted steering device and to a hydraulic motor of an engine cooling fan.
2. Discussion of the Background
A conventional hydraulic tandem pump assembly of this kind includes a primary pump for the power-assisted steering device and a secondary pump for the hydraulic motor of the engine cooling fan, which pumps are mounted on a common drive shaft in drive connection to an output shaft of a prime engine of the vehicle. In operation, both the pumps are simultaneously driven by rotation of the common drive shaft to supply fluid under pressure therefrom to the power-assisted steering device and to the hydraulic motor, even in a cold season as well as in a warm season. In the cold season, the viscosity of hydraulic fluid increases to cause an increase of fluid resistance in pipe lines of the pumps. This results in an increase of internal pressure in the pumps, causing power loss of the prime engine. As the secondary pump has a large displacement capacity for supplying a sufficient amount of fluid under pressure to the hydraulic motor of the engine cooling fan, the power loss of the prime engine becomes large in operation of the hydraulic motor. When the power assisted-steering device is operated to steer the vehicle without driving, the pressure in the primary pump increases to render the entirety of the tandem pump assembly in a heavy loaded condition. In such a condition, there is a fear of causing jam of the pumps.